Situations have developed in laboratories and in industry, where operations over a wide range of temperatures are mandated, and very rapid changes between extremes of temperature may provide advantages that enhance results and provide economies of operation.
For example, gas chromatographs are used to determine the quantities of chemical components making up a liquid or gas sample, and cryogenic cooling is used for several purposes associated with gas chromatographs. For chromatography, a gas or liquid sample is fed through a cryotrap at cryogenic temperatures to freeze out unwanted contaminants, the most common being water, and the remainder of the sample flows from the trap and into the gas chromatograph for analysis. After the analysis is completed, the temperature of the cryotrap is raised to an elevated temperature, which may be as high as 400.degree. C., to purge the trap of contaminants prior to evaluating another sample. It is advantageous in a series of operations if cool-down and heating times are minimized.
Sometimes, in addition to a cryotrap, a cryofocuser, also referred to as a cryoconcentrator, is used to condense the constituents of interest so that they are concentrated inside the cryofocuser. Then, the temperature of the cryofocuser is raised rapidly to an elevated level to quickly release the accumulated components of interest. This sequence effectively raises the sensitivity of the analysis by concentrating the target components over a longer time period than the time period for delivery to the chromatograph.
Cryofocusers generally operate in a range between -200.degree. C. and 400.degree. C. In both instances, cryogenic cooling is accomplished by evaporation of liquid nitrogen.
A second device called a Dynamic Material Analyzer (DMA) is used to characterize the stiffness and damping of material samples by imposing sinusoidal deflections on one side of the sample and measuring the resultant force transmitted to the other side of the sample. Frequency is often varied to determine the changes in material stiffness and damping that occur with frequency. Many DMA's also have the capability to measure material properties for temperatures ranging from -150.degree. C. to 600.degree. C. Again, cryogenic cooling is currently accomplished by evaporating liquid nitrogen.
Differential Scanning Calorimeters (DSC) determine the heat capacity of materials as a function of temperature by measuring the rate of temperature change of the sample for a known heating rate and sample mass. The temperature range of interest is frequently -100.degree. C. to 750.degree. C., and again, cooling is currently accomplished by use of liquid nitrogen.
Attributes common to the three applications are the need to cool samples to cryogenic temperatures, to perform measurements over a broad range of temperatures, and the need to vary the temperature up or down rapidly. As stated, the current method of cooling in all three applications is the use of liquid nitrogen; on the other hand, sample temperature is increased by powering an electric heater.
In a typical cryotrap function for a gas chromatograph, liquid nitrogen flows into a chamber around an object to be cooled; the object may be a capillary tube. A resistance wire heater is wrapped around the capillary tube. Temperatures between room temperature and the boiling temperature of liquid nitrogen (-196.degree. C.) are attained by a combination of modulating electrical power to the heater and pulsing the flow of liquid nitrogen on and off. Temperatures above room temperature are attained by not flowing liquid nitrogen, and simply powering the heater. It is a crude method of temperature control, especially since continuous modulation of N.sub.2 flow using automatic thermo-mechanical devices is not reliable or easily achieved. Liquid carbon dioxide (boiling temperature -78.degree. C.) is also sometimes used as a refrigerant instead of liquid nitrogen.
With such apparatuses, and in other laboratory test equipment involving wide ranges of temperature, it is most desirable that changes in temperature can be effected rapidly. The rapidity of temperature change depends upon the required range of temperature and the mass that is being heated and cooled during operation. Thus, the structure that supports, for example, a test sample in a gas chromatograph, is important as its mass may be greater than the mass of the sample to be tested. In such an application, the cooling and heating requirements are small for the sample and relatively substantial for the apparatus. The total mass should be minimized so that the cool-down time and warm up times are low. Heating and cooling a large mass not only increases the response time but also increases the requirements for liquid refrigerant that is evaporated during the cooling process and that may be used as a modulator during the heating process.
Also, to keep the cooling load low at the cryogenic temperature interface, it is necessary to isolate the heating source from the cooling source as much as possible. When complete isolation is not possible, excessive heat reaches the cooling source while heating a test sample and a larger heat removal capacity than at cryogenic temperatures is required, especially when cooling continues during heating. On the other hand, if cooling is entirely shut down during heating operations, the structure is warmed and the response time is very slow when cooling is again required.
For these reasons, a closed cycle cryocooler system has not been used in chromatograph, DMA and DSC applications. A closed cycle cryocooler that is adequate solely for cooling would be overwhelmed by unwanted heat, conducted through the sample, during heating of the sample. As a result, the cryocooler's cold head temperature would rise; its response time when the sample was to be cooled would be slow.
Using a sapphire to make a thermal interface for cryogenic refrigerators is not a novel concept. The assignee of the present application has been using sapphire interfaces since 1983, and manufactures and offers for sale sapphire interfaces for laboratory experiments. Others also sell a sapphire interface.
Sapphire has unusual, but well known properties, namely a high thermal conductivity at cryogenic temperature and a low thermal conductivity at elevated temperature. The thermal conductivity of sapphire versus temperatures is compared with that of stainless steel and copper in FIGS. 1a,b.
In use as an interface, the object to be temperature-controlled is placed at one end of a sapphire rod together with a heater. The cold tip (cold head) of a commercially available closed cycle cooler is placed at the opposite end of the sapphire rod. When the temperature of the object is raised by the heater, the temperatures at the heater-end of the sapphire and of the entire assembly are also somewhat elevated. Warming the sapphire lowers its thermal conductivity, and the amount of heat which reaches the closed cycle cooler by conduction through the sapphire is thereby not linearly related to the increasing temperature difference between the ends of the sapphire. Since the thermal conductivity of sapphire decreases rapidly with temperature increase, the sapphire effectively acts as a control that limits the amount of heat transferred to the closed cycle cooler. Therefore, a smaller refrigeration cycle can operate without overloading or wide swings in its cold temperature during warming of the test object, than would be feasible without the intermediate sapphire interface.
As is known, the amount of heat which the sapphire element (or any solid rod) will conduct from one end to the other is related directly to the thermal conductivity of the element, cross sectional area of the element, element length, and the temperature difference from one end of the element to the other end (length). The amount of heat transferred is inversely related to the length of the element.
Thus, in determining the quantity of heat that will flow through the sapphire rod, thermal conductivity is a variable dependent upon temperature, and temperature is a variable. Length and cross sectional area are fixed by design. The amount of heat that will be transferrable by the sapphire rod at any given conditions is therefore determined by the product (integral) of thermal conductivity (at that time) and temperature differential across the element (at that time).
FIG. 2 displays the mathematical product of thermal conductivity and differential temperature AT for four materials of interest, individually normalized by dividing by the maximum value of the product for each material respectively over the temperature range of interest. In FIG. 2, it is assumed that the cold temperature T.sub.c, for example, at one end of the sapphire, remains constant at 80K. The "heat station" is the opposite end of the element where the heater would typically be mounted and in heating a maximum temperature T.sub.h of 800K. is reached. The sapphire displays a desirable property of the product of thermal conductivity and differential temperature, which product is substantially constant over a major portion of the entire temperature range. This characteristic is in contrast to the heat transfer rate (product) of copper and stainless steel, which materials show poor heat transfer at the low temperatures and high heat transfer at high temperatures. A second material, quartz, which is usable in place of sapphire, also has a relatively constant heat transfer rate. Materials showing an upward arc in a graph such as FIG. 2 are generally usable with advantage in a thermal interface. Presently known, sapphire has the highest performance potential. Quartz is acceptable in many applications. Materials having k.DELTA.T properties between quartz and sapphire, such as single crystal silicon, are good alternatives to sapphire. Other suitable materials including ones superior to sapphire, may be developed in the future and are considered to fall within the scope of the present invention.
With regard to FIG. 2, which is a normalized graph of the thermal conductivity integral versus temperature, it is easy to identify which materials provide an advantage in a thermal interface in accordance with the invention. It is apparent, whether or not the thermal conductivity changes in a manner that reduces the load at the system cold head when the test subject is being heated, and the cooling system is simultaneously operated.
A material having a thermal conductivity that is independent of temperature changes, would plot on such a graph as a straight line between the selected temperatures of T.sub.c 80K. and T.sub.h 800K. The slope of that line would be 1/(T.sub.h -T.sub.c). Any material that graphically "bulges" upward from that line, such as quartz and sapphire in FIG. 2, would be favorable for the thermal interface in accordance with the invention and would be characterized by a slope at T.sub.c (80K.) greater than the slope 1/(T.sub.h -T.sub.c). Also, the slope of the "bulging" line at T.sub.h (800K.) would be less than the slope 1/(T.sub.h -T.sub.c). Materials having a characteristic that on a graph similar to FIG. 2 falls below a straight line connecting T.sub.c and T.sub.h, would be unfavorable for use in the present invention. The load on the cold head would increase unfavorably when the test subject was heated and would require a larger cooling system if successful operations are to be achieved as compared to the upper-bulging type materials.
It should be understood, that a graph similar to FIG. 2 can also be provided without normalizing the values of the ordinate. This will result in a wide range of values plotted for the ordinate, whereas, when normalized, the ordinate always has a range from 0-1. Normalizing makes for easier comparison of materials. It can be readily seen in FIG. 2 that when cooling from 800.degree. to 80.degree. K., the sapphire will provide the greatest capacity for cooling and therefore the quickest cooldown.
Nevertheless, the known sapphire interface only partially resolves the problems associated with heating and cooling of a sample over a wide range in that the commercially available interfaces have high mass and therefore relatively low response time. As a result a sapphire interface and closed cycle refrigerator have not been applied in gas chromatographs, DMAs and DSCs.
It is generally desirable in these applications to heat up and cool-down from temperature extremes in a matter of minutes, whereas, cool-down time for commercially available closed cycle coolers ranges from 15 minutes to hours. Thus, to be useful in these special apparatuses, a closed cycle cooler must remain cold while the heat station with the test object or sample is warmed in order to subsequently achieve rapid cooling at the heat station.
The amount of heat which is allowed to flow into the cooler while a test object is heated must be limited to prevent the cooler from warming up. With conventional materials like copper and stainless steel, maximum heat transfer takes place when the heat station is at maximum temperature, as shown in FIG. 2, and the heat transfer rate drops off rapidly from that point. For sapphire, the heat transfer rate is close to maximum across the whole temperature range of interest. The higher the heat transfer rate during cool-down, the faster the cool-down. Since the heat transfer rate for sapphire remains close to maximum throughout the temperature range, a sapphire interface can provide the most rapid cooling rate.
What is needed, is a heating/cooling wide temperature range interface that eliminates the need for expendable liquid refrigerant, provides rapid response times and satisfies these conditions with low maximum cooling requirements.